Plasmonic funnel for focused optical delivery to a metallic medium

ABSTRACT

An apparatus includes a transducer including a plasmonic funnel having first and second ends with the first end having a smaller cross-sectional area than the second end, and a first section positioned adjacent to the first end of the plasmonic funnel, and a first waveguide having a core, positioned to cause light in the core to excite surface plasmons on the transducer.

BACKGROUND

Heat assisted magnetic recording (HAMR) generally refers to the conceptof locally heating a recording medium to reduce the coercivity of themedium so that an applied magnetic writing field can more easily directthe magnetization of the medium during the temporary magnetic softeningof the medium caused by the heat source. The heated area in the storagelayer determines the data bit dimension. A tightly confined, high powerlight spot is used to heat a portion of the recording medium tosubstantially reduce the coercivity of the heated portion. Then theheated portion is subjected to a magnetic field that sets the directionof magnetization of the heated portion. In this manner the coercivity ofthe medium at ambient temperature can be much higher than the coercivityduring recording, thereby enabling stability of the recorded bits atmuch higher storage densities and with much smaller bit cells. Heatassisted magnetic recording is also referred to a thermally assistedmagnetic recording.

Near-field transducers can be used to focus light to a small spot. Anefficient means for concentrating light with a near-field transducerwould be beneficial in HAMR recording heads.

SUMMARY

In one aspect, the disclosure provides an apparatus including atransducer including a plasmonic funnel having first and second endswith the first end having a smaller cross-sectional area than the secondend, and a first section positioned adjacent to the first end of theplasmonic funnel, and a first waveguide having a core, positioned tocause light in the core to excite surface plasmons on the transducer.

These and other features and advantages which characterize the variousembodiments of the present disclosure can be understood in view of thefollowing detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of portions of a recording headincluding an embodiment.

FIG. 2 is a schematic representation of portions of another recordinghead including an embodiment.

FIG. 3 is a cross-sectional view of FIG. 2 with an additional magneticpole.

FIGS. 4-8 are schematic cross-sectional views of several examples ofplasmonic funnels.

FIGS. 9 and 10 are schematic representations of a portion of adielectric channel waveguide and a mode converter in an embodiment.

FIG. 11 is a schematic representation of portions of another recordinghead including an embodiment.

FIG. 12 is a schematic representation of portions of another recordinghead including an embodiment.

FIGS. 13-15 are schematic cross-sectional views of several examples ofplasmonic funnels.

FIG. 16 is a schematic representation of portions of another recordinghead including an embodiment.

FIG. 17 is a schematic representation of portions of another recordinghead including an embodiment.

FIG. 18 is a schematic representation of portions of another recordinghead including an embodiment.

FIG. 19 is a graph of an electric field component of light in thewaveguide of FIG. 18.

FIG. 20 is a pictorial representation of a data storage device in theform of a disc drive that can include a recording head in accordancewith an aspect of this disclosure.

DETAILED DESCRIPTION

In one aspect, this disclosure provides an apparatus for focusing lightto a small spot. In one embodiment, light is focused from a firstwaveguide to a recording medium such that the focused spot is muchsmaller than the diffraction limit.

FIG. 1 is a schematic representation of a portion of a recording head 10that may be used in a heat assisted magnetic storage device. Therecording head includes a first magnetic pole 12, which in this exampleis a write pole, a first waveguide in the form of a dielectric channelwaveguide 14 that includes a core layer 16, and a tapered blockplasmonic transducer 18 positioned between a portion of the core layerof the waveguide and the pole. The core is positioned to cause light inthe core to excite surface plasmons on the transducer. The core of thedielectric channel waveguide has a substantially rectangularcross-sectional shape in this example. The recording head is positionedadjacent to a data storage medium 20, which can be a metallic medium.The recording head may be separated from the storage medium by an airbearing 22.

In the example of FIG. 1, the plasmonic transducer includes a firstsection 24 (also referred to as a plasmonic waveguide or strip) having asubstantially rectangular cross-sectional shape and having substantiallystraight sides that lie in planes substantially perpendicular to a planeof the storage medium; a second section 26 (also referred to as a secondplasmonic waveguide or strip) having a substantially rectangularcross-sectional shape and having substantially straight sides that liein planes substantially perpendicular to a plane of the storage medium;and a plasmonic funnel section 28 positioned between the first andsecond plasmonic waveguides. The plasmonic funnel section includes afirst (or bottom) end positioned adjacent to the first section, and asecond (or top) end positioned adjacent to the second section, with thecross-sectional area of the first end being smaller than thecross-sectional area of the second end.

The plasmonic funnel transducer serves as a near-field transducer (NFT)and includes a tapered section that is tapered to concentrate plasmonsin a direction toward the first plasmonic waveguide. In this example,the plasmonic funnel section includes two substantially flat sides thatare tilted to form edges that meet edges of the sides of the first andsecond plasmonic waveguides, and two other substantially flat sides thatlie in planes that are substantially perpendicular to the plane of thestorage medium and form edges that meet edges of the sides of the firstand second plasmonic waveguides. In FIG. 1 only one tilted side 30 andonly one flat side 32 of the tapered portion are visible. An end 34 ofthe pole and an end 36 of the plasmonic transducer are positionedadjacent to an air bearing surface 38 of the recording head.

A portion 40 of the dielectric channel waveguide core is positionedadjacent to a side 42 of the second plasmonic waveguide such that lightin the waveguide core is evanescently coupled to the plasmonictransducer. A gap 44 can be provided between portion 40 and the side 42.The adjacent portions of the dielectric channel waveguide core and theside of the second plasmonic waveguide form a dielectric waveguide modeto plasmonic mode coupler. In operation, light 46 from a light sourcesuch as a laser 48, propagates through the dielectric channel waveguideand excites local surface plasmons on the plasmonic transducer. Plasmonsare concentrated by the plasmonic funnel as they travel from the secondplasmonic waveguide (i.e., the second section of the transducer) to thefirst plasmonic waveguide (i.e., the first section of the transducer).

Light in the dielectric channel waveguide propagates through thedielectric channel waveguide in an incident mode. The plasmonictransducer 18 is positioned next to the core of the dielectric channelwaveguide. The tapered block plasmonic funnel and the first and secondplasmonic waveguides are made of a plasmonic material. The plasmonicmaterial can be, for example, gold, silver, copper, aluminum, or alloysof these materials.

The magnetic write pole 12 is placed behind (or adjacent to) the taperedblock plasmonic transducer. The pole can be straight as shown in FIG. 1,or in other embodiments, the pole can be sloped toward the plasmonicfunnel, or stacked. The region 49 surrounding the transducer, waveguidecore, and magnetic pole of FIG. 1 can be filled with a dielectricmaterial, that can serve as cladding layers for the dielectricwaveguide. The dielectric material can be, for example, silica, siliconoxynitride, alumina, tantala, magnesium oxide, silicon nitride, ortitania. In the example of FIG. 1, both the first and second plasmonicwaveguides have a rectangular cross-sectional shape in a planesubstantially parallel to the plane of the recording medium.

FIG. 2 is a schematic representation of portions of another recordinghead 50, which includes many of the elements of the recording head ofFIG. 1, and further includes a plasmonic shield 52 that is positionedadjacent to a side of the first section of the plasmonic funneltransducer opposite the first magnetic pole.

FIG. 3 is a cross-sectional view of the recording head of FIG. 2 showinga second magnetic pole 54. In this example, light illustrated by awavefront 56 travels through the dielectric channel waveguide core in atransverse magnetic (TM) waveguide mode, with an electric field orientedas shown by arrow 58. This light excites surface plasmons 60 on thetapered block plasmonic funnel transducer. The recording head is spacedfrom the data storage medium by an air bearing 62. The plasmons areconcentrated by the plasmonic funnel into a small spot adjacent to anair bearing surface 64 of the recording head.

The plasmonic front shield is shown to be positioned in front of theplasmonic funnel transducer in FIG. 3. There is a small gap 66 betweenthe plasmonic funnel and the front shield. The gap can be filled withdielectric materials of various refractive indices. Such dielectricmaterials include, for example, silica, silicon oxynitride, alumina,tantala, magnesium oxide, silicon nitride, or titania. The materialselection can be based on the optical spot size created by the NFT,optical loss in the NFT, and/or the efficiency of coupling to the media.

The magnetic writer has two poles. The first (or main) pole 12 is shownin FIG. 2. The second pole 54 can either be placed behind the main pole,or in front of the plasmonic front shield, and in contact with plasmonicfront shield, as shown in FIG. 3. The roles of the main pole and thesecond pole can be reversed in the sense that either one can be madenarrower than the other in the cross-track direction (i.e., theY-direction).

The plasmonic shield reduces the dissipation of the plasmonic mode intothe magnetic poles and reduces the optical spot curvature in the medium.In addition, the front shield screens the second pole from the plasmonicfields.

FIG. 3 can be used to explain the operation of the device. Initially,light propagates in a TM mode in the dielectric channel waveguide core.The electric field polarization direction is shown in FIG. 3. The energyin this mode is transferred into a surface plasmonic mode that runsalong the face/edge 68 of the tapered block plasmonic funnel transducer.As the mode propagates towards the storage medium, the mode confinementbecomes smaller and smaller due to the taper in the plasmonic funnel.Finally, the funnel ends in a narrow straight strip 36 (referred to asthe first section or the first plasmonic waveguide in FIG. 1). The stripcan be long enough (in a direction substantially perpendicular to thestorage medium) so that the fringing field at the junction between thetapered funnel portion and the straight strip do not interact with themedium. This ensures a low sensitivity to lapping during fabrication ofthe recording head. Due to light delivery constraints, it might beeasier to launch a transverse electric (TE) mode in the dielectricchannel waveguide. In this case, the TE mode can be converted to thesurface plasmon mode on the edge of the plasmonic funnel in the samemanner as with the TM mode in FIG. 3, or the TE dielectric channelwaveguide mode can be converted into a TM dielectric channel waveguidemode as described below.

FIG. 4 shows the desired phase of the surface plasmon mode on theplasmonic funnel, with plus and minus signs illustrating relative phase.If a TE mode is converted to the surface plasmon mode on the edge of thetapered block plasmonic funnel transducer, the plasmon mode will have anopposite phase on either side of the plasmonic funnel transducer, asillustrated in FIG. 5. This will not produce a confined optical spotunder the plasmonic funnel transducer at the recording medium. Tocorrect for this phase difference, a 180° phase shift can be introducedon one side of the plasmonic funnel. This can be achieved by using atuning stub 70 as shown in FIG. 6, or by extending the path on one sideto obtain the desired optical path difference between the two sides. Thepath can alternatively be extended by a groove 72 in one side as shownin FIG. 7, or by a protrusion 74 on one side as shown in FIG. 8.

In another example, the TE dielectric channel waveguide mode can beconverted into a TM dielectric channel waveguide mode using a previouslyknown technique in which two plasmonic studs of appropriate length areplaced diagonally with respect to the core of the dielectric channelwaveguide. A structure for implementing this approach is shown in FIGS.9 and 10, wherein FIG. 9 is a side view and FIG. 10 is a cross-sectionalview. Two plasmonic studs 80 and 82 are placed at diagonally oppositepositions with respect to the channel waveguide core. This introduces across-talk between the TE and the TM modes. By choosing a suitablelength of the studs, the light that is launched as a TE mode can beconverted to a TM mode. The incident electric field component of thelight is shown as arrow 84 and the exiting electric field component ofthe light is shown as arrow 86. Light travels in the direction of arrow88.

Alternatively, the mode coupling method of surface plasmon launching canbe replaced with an end fire method. In the method discussed above, theenergy transfer takes place across a gap between a portion of the coreof the dielectric waveguide and the plasmonic funnel transducer.Alternatively, these modes can be launched using end-fire coupling. Theend fire method can be implemented using the structure shown in FIG. 11.

FIG. 11 shows a plasmonic funnel transducer 90 including a first taperedsection 92, a center section 94 having a substantially rectangularcross-sectional shape, a second tapered section 96 and a strip section98 having a substantially rectangular cross-sectional shape. A corelayer 100 of a dielectric channel waveguide 102 overlaps an end 104 ofthe plasmonic funnel transducer in the downtrack direction (i.e., theX-direction). When used in a recording head, the end 106 of the secondrectangular section would be positioned adjacent to an air bearingsurface 108. A small vertical gap 110 (normal to the air bearing surfaceof the recording head) can be included between the plasmonic funnel andthe channel waveguide core. The funnel in FIG. 11 is tapered at the topto provide for a gradual mode transfer from the dielectric channelwaveguide to the plasmonic funnel. With the top taper in place, thesmall vertical gap 110 can be reduced to zero or be made negative (i.e.the plasmonic funnel can be penetrated by the waveguide core).

FIG. 12 is a schematic representation of portions of another recordinghead 120. The recording head of FIG. 12 includes a plasmonic funneltransducer 90 as shown in FIG. 11, in combination with a channelwaveguide core 122. The waveguide core is vertically aligned with theplasmonic funnel transducer such that light exiting the end 124 of thecore is end fire coupled to the plasmonic funnel transducer. The end ofthe core can be separated from the plasmonic funnel transducer by a gap126. Alternatively, the end 124 can be in contact with the plasmonicfunnel transducer or embedded in it.

FIGS. 13-15 are schematic cross-sectional views of several examples ofplasmonic funnel transducers 130, 132 and 134. Plasmonic funneltransducer 130 includes a first plasmonic waveguide 136 and a secondplasmonic waveguide 138. A tapered section 140 is positioned between thefirst and second plasmonic waveguides. The tapered section includesconcave sides 142 and 144. The first and second plasmonic waveguides,and the tapered section, can have generally rectangular cross-sectionalshapes in planes perpendicular to the plane of the drawing. In variousembodiments, the tapered section can include two concave sides and twoflat sides, or four concave sides.

Plasmonic funnel transducer 132 includes a first plasmonic waveguide 146and a second plasmonic waveguide 148. A tapered section 150 ispositioned between the first and second plasmonic waveguides. Thetapered section includes convex sides 152 and 154. The first and secondplasmonic waveguides, and the tapered section, can have generallyrectangular cross-sectional shapes in planes perpendicular to the planeof the drawing. In various embodiments, the tapered section can includetwo convex sides and two flat sides, or four convex sides.

Plasmonic funnel transducer 134 includes a first plasmonic waveguide 156and a second plasmonic waveguide 158. A tapered section 160 ispositioned between the first and second plasmonic waveguides. Thetapered section includes convex sides 162 and 164. The first and secondplasmonic waveguides, and the tapered section, can have generallyrectangular cross-sectional shapes in planes perpendicular to the planeof the drawing. In various embodiments, the tapered section can includetwo convex sides and two flat sides, or four convex sides.

FIG. 16 is a schematic representation of portions of another recordinghead 170. The recording head includes a first magnetic pole 172, whichin this example is a write pole and a plasmonic funnel transducer 174positioned adjacent to the pole. The recording head is positionedadjacent to a data storage medium 176. The recording head may beseparated from the storage medium by an air bearing 178.

In the example of FIG. 16, the plasmonic funnel transducer includes afirst plasmonic waveguide 180 having a substantially rectangularcross-sectional shape and having substantially straight sides that liein planes substantially perpendicular to a plane of the storage mediumand a tapered funnel portion 182 positioned above the first plasmonicwaveguide. Plasmonic side shields 184 and 186 are positioned on oppositesides of the first plasmonic waveguide and adjacent to the air bearingsurface of the recording head.

The plasmonic funnel is tapered to concentrate plasmons in a directiontoward the first plasmonic waveguide. In this example, the plasmonicfunnel includes two substantially flat sides that are tilted to formedges that meet edges of the sides of the first plasmonic waveguide, andtwo other substantially flat sides that lie in planes that aresubstantially perpendicular to the plane of the storage medium and formedges that meet edges of the sides of the first plasmonic waveguides. InFIG. 16 only one tilted side 188 and only one flat side 190 of thetapered portion are visible. An end 192 of the pole and an end 194 ofthe plasmonic transducer are positioned adjacent to an air bearingsurface 196 of the recording head. In another embodiment, the sideshields can be used in combination with a front shield as shown in FIG.2.

FIG. 17 is a schematic representation of portions of another recordinghead 200. The recording head of FIG. 17 includes a planar waveguide 202in the form of solid immersion mirror (SIM) having a core layer 204. Thesides 206 and 208 of the solid immersion mirror are shaped to directlight in the core to a focal point 210. The core layer is positionedsuch that light exiting the core layer excites surface plasmons on theplasmonic funnel transducer 90. The SIM core layer can be verticallyaligned with the plasmonic funnel transducer for end fire coupling, orthe core can be offset from the plane of the plasmonic funnel transducerfor evanescent coupling.

FIG. 18 is a schematic representation of portions of another recordinghead 220. The recording head of FIG. 18 includes a waveguide 222 in theform of mode index lens 224 having a core layer 226 with a first region228 and a second region 230 having a thickness larger than the thicknessof the first region. The edge of the second region is shaped to directlight in the core to a focal point 230. The mode index lens ispositioned such that light exiting the core layer excites surfaceplasmons on the plasmonic funnel transducer 90. The mode index lens canbe vertically aligned with the plasmonic funnel transducer for end firecoupling, or it can be offset from the plane of the plasmonic funneltransducer for evanescent coupling.

FIG. 19 is a graph of an electric field component of light in thewaveguide of FIG. 18. To obtain the desired phase of light exiting themode index lens, the electric field component of light region 228 can beasymmetric with respect to the center of the mode index lens in a planeperpendicular to the plane of the drawing. The desired electric field isshown in the region between the arrows 232 and 234 in FIG. 19. If thedesired phase is not obtained in this manner, the tuning stubs shown inFIGS. 9 and 10 could be used to obtain the desired phase.

In FIGS. 1-18, only selected components of the apparatus are shown. Itwill be apparent to those skilled in the art that other components canbe included in a practical device. For example, the components in FIGS.1-18 can be embedded in or surrounded by material, which may bedielectric material, that supports the illustrated components andmaintains the relative position of the illustrated components.

FIG. 20 is a pictorial representation of a magnetic storage device inthe form of a disc drive that can include a recording head constructedin accordance with the disclosure. The disc drive 240 includes a housing242 (with the upper portion removed and the lower portion visible inthis view) sized and configured to contain the various components of thedisc drive. The disc drive 240 includes a spindle motor 244 for rotatingat least one storage medium 246, which may be a magnetic recordingmedium, within the housing 242. At least one arm or other positioningdevice 248 is contained within the housing 242, with each arm 248 havinga first end 250 with a recording head or slider 252, and a second end254 pivotally mounted on a shaft by a bearing 256. An actuator motor 258is located at the arm's second end 254 for pivoting the arm 248 toposition the recording head 252 over a desired sector or track 260 ofthe disc 246. The actuator motor 258 is regulated by a controller, whichis not shown in this view and is well-known in the art.

For heat assisted magnetic recording (HAMR), an electromagnetic wave of,for example, visible, infrared or ultraviolet light is directed onto asurface of a data storage medium to raise the temperature of a localizedarea of the medium to facilitate switching of the magnetization of thearea. The recording head can include a laser, channel waveguide, andplasmonic funnel transducer as shown in FIGS. 1-18 on a slider to guidelight to the storage medium for localized heating of the storage medium.

The various examples described above include a dielectric waveguide modeto plasmonic mode coupler, a plasmonic funnel transducer, and a narrowplasmonic waveguide or strip positioned adjacent to a tapered section.An optional polarization rotator can also be included. The apparatus canbe used for light delivery in heat assisted magnetic recording. It canalso be used in other applications that require coupling between twowaveguides.

In one aspect, the disclosure provides a transducer including aplasmonic funnel including first and second ends with the first endbeing narrower than the second end, a first plasmonic waveguidepositioned adjacent to the first end of the plasmonic funnel, and asecond plasmonic waveguide positioned adjacent to the second end of theplasmonic funnel, wherein a phase changing element is positioned along aside of the second plasmonic waveguide. The phase changing element canbe, for example, a stub, an indent, or a protrusion.

In another aspect, the disclosure provides an apparatus including arecording medium; a recording head having a transducer including aplasmonic funnel having first and second ends with the first end havinga smaller cross-sectional area than the second end, and a first sectionpositioned adjacent to the first end of the plasmonic funnel, and afirst waveguide having a core, with a portion of the core positioned tocause light in the core to excite surface plasmons on the transducer;and a positioning means for positioning the recording head adjacent tothe storage medium. A portion of the first waveguide core can bepositioned adjacent to a side of the transducer such that light in thewaveguide core is evanescently coupled to the transducer. The transducercan also include a second section positioned adjacent to the second endof the plasmonic funnel. An end of the first waveguide core can bepositioned adjacent to an end the transducer such that light in thewaveguide core is end fire coupled to the transducer.

It is to be understood that even though numerous characteristics andadvantages of various embodiments of the present invention have been setforth in the foregoing description, together with details of thestructure and function of various embodiments of the invention, thisdetailed description is illustrative only, and changes may be made indetail, especially in matters of structure and arrangements of partswithin the principles of the present invention to the full extentindicated by the broad general meaning of the terms in which theappended claims are expressed. For example, the particular elements mayvary depending on the particular application without departing from thespirit and scope of the present invention.

What is claimed is:
 1. An apparatus comprising: a transducer including aplasmonic funnel including first and second ends with the first endhaving a smaller cross-sectional area than the second end, and a firstsection positioned adjacent to the first end of the plasmonic funnel;and a first waveguide having a core, positioned to cause light in thecore to excite surface plasmons on the transducer.
 2. The apparatus ofclaim 1, wherein a portion of the first waveguide core is positionedadjacent to a side of the transducer such that light in the waveguidecore is evanescently coupled to the transducer.
 3. The apparatus ofclaim 1, wherein the transducer further comprises: a second sectionpositioned adjacent to the second end of the plasmonic funnel.
 4. Theapparatus of claim 3, wherein an end of the first waveguide core ispositioned adjacent to an end the transducer such that light in thewaveguide core is end fire coupled to the transducer.
 5. The apparatusof claim 3, wherein the second section includes first and second endswith the first end having a smaller cross-sectional area than the secondend, with the second end being positioned adjacent to the second end ofthe plasmonic funnel.
 6. The apparatus of claim 3, further comprising: aphase changing element positioned along a side of the second section. 7.The apparatus of claim 6, wherein the phase changing element comprisesone of: a stub, an indent, or a protrusion.
 8. The apparatus of claim 1,wherein the first waveguide comprises one of: a channel waveguide, asolid immersion mirror, or a mode index lens.
 9. The apparatus of claim8, further comprising: a polarization rotator adjacent to the firstwaveguide.
 10. The apparatus of claim 9, wherein the polarizationrotator comprises one of: first and second plasmonic stubs adjacent todiagonally opposite edges of the dielectric channel waveguide.
 11. Theapparatus of claim 1, further comprising: a first magnetic polepositioned adjacent to a side of the first section of the transducer.12. The apparatus of claim 11, further comprising: a second magneticpole; and a plasmonic shield positioned between the second magnetic poleand the transducer.
 13. The apparatus of claim 12, wherein a gap betweenthe plasmonic shield and the transducer is filled with dielectricmaterial.
 14. The apparatus of claim 1, further comprising: first andsecond plasmonic shields positioned adjacent to opposite sides of thefirst portion.
 15. An apparatus comprising: a transducer including aplasmonic funnel including first and second ends with the first endbeing narrower than the second end, a first plasmonic waveguidepositioned adjacent to the first end of the plasmonic funnel, and asecond plasmonic waveguide positioned adjacent to the second end of theplasmonic funnel; and a phase changing element positioned along a sideof the second plasmonic waveguide.
 16. The apparatus of claim 15,wherein the phase changing element comprises one of: a stub, an indent,or a protrusion.
 17. An apparatus comprising: a recording medium; arecording head having a transducer including a plasmonic funnel havingfirst and second ends with the first end having a smallercross-sectional area than the second end, and a first section positionedadjacent to the first end of the plasmonic funnel, and a first waveguidehaving a core, positioned to cause light in the core to excite surfaceplasmons on the transducer; and a positioning means for positioning therecording head adjacent to the storage medium.
 18. The apparatus ofclaim 17, wherein a portion of the first waveguide core is positionedadjacent to a side of the transducer such that light in the waveguidecore is evanescently coupled to the transducer.
 19. The apparatus ofclaim 17, wherein the transducer further comprises: a second sectionpositioned adjacent to the second end of the plasmonic funnel.
 20. Theapparatus of claim 19, wherein an end of the first waveguide core ispositioned adjacent to an end the transducer such that light in thewaveguide core is end fire coupled to the transducer.